Mixed metal oxide and sodium secondary battery

ABSTRACT

The present invention provides a sodium secondary battery capable of reducing the amount of lithium used, and ensuring a larger discharge capacity maintenance rate when having repeated a charge and discharge, as compared with conventional techniques; and a mixed metal oxide usable as the positive electrode active material therefor. A mixed metal oxide of the present invention comprises Na and M 1  wherein M 1  represents three or more elements selected from the group consisting of Mn, Fe, Co and Ni with an Na:M 1  molar ratio being a:1 wherein a is a value falling within the range of more than 0.5 and less than 1. Also, a mixed metal oxide of the present invention is represented by the following formula (1):
 
Na a M 1 O 2   (1)
 
wherein M 1  and a each have the same meaning as above.
 
     The positive electrode active material for secondary batteries of the present invention comprises the mixed metal oxide above.

TECHNICAL FIELD

The present invention relates to a mixed metal oxide and a sodiumsecondary battery.

BACKGROUND ART

A mixed metal oxide is being used as a positive electrode activematerial of a secondary battery. Among secondary batteries, a lithiumsecondary battery has already been put into commercial use as a smallpower source for cellular phones, notebook computers and the like.Furthermore, because of its applicability as a large power source, forexample, as a power source for vehicles such as electric vehicle andhybrid electric vehicle, or as a power source for distributed powerstorages, the demand thereof is on the rise. However, in a lithiumsecondary battery, a large amount of scarce metal elements such aslithium are contained in the raw material of the positive electrodeactive material, and there is concern about supply of the raw materialto meet the growing demand for a large power source.

In response, a sodium secondary battery is being studied as a secondarybattery capable of eliminating the concern about supply. The sodiumsecondary battery can be fabricated using a material which has aplentiful supply and which is inexpensive, and its commercialapplication is expected to allow for a large supply of large powersources.

In Patent Document 1, a positive electrode active material obtained bycalcining a raw material comprising Na, Mn and Co in a compositionalratio (Na:Mn:Co) of 0.7:0.5:0.5 is specifically described as thepositive electrode active material for sodium secondary batteries.

Patent Document 1: Japanese Unexamined Patent Publication No.2007-287661 (Example 1)

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

The amount of lithium used can be reduced by using the above-describedpositive electrode active material, but in a sodium secondary batteryusing this positive electrode active material, the discharge capacitymaintenance rate when having repeated a charge and discharge is notsufficient. An object of the present invention is to provide a sodiumsecondary battery capable of reducing the amount of lithium used, andensuring a larger discharge capacity maintenance rate when havingrepeated a charge and discharge as compared with conventionaltechniques; and a mixed metal oxide usable as the positive electrodeactive material therefor.

Means to Solve the Problem

The present inventors have continued intensive studies to attain theabove-described object, and accomplished the present invention. That is,the present invention provides the following inventions.

<1> A mixed metal oxide comprising Na and M¹ (wherein M¹ representsthree or more elements selected from the group consisting of Mn, Fe, Coand Ni, with an Na:M¹ molar ratio being a:1 wherein a is a value fallingwithin the range of more than 0.5 and less than 1.

<2> A mixed metal oxide represented by the following formula (1):Na_(a)M¹O₂  (1)

wherein M¹ and a each have the same meaning as above.

<3> The mixed metal oxide as described in <1> or <2> above, wherein M¹comprises at least Mn.

<4> The mixed metal oxide as described in <3> above, wherein an M¹:Mnmolar ratio is 1:b wherein M¹ has the same meaning as above, and b is avalue of not less than 0.2 and less than 1.

<5> The mixed metal oxide as described in any one of <1> to <4> above,wherein M¹ represents Mn, Fe and Ni.

<6> The mixed metal oxide as described in any one of <1> to <5> above,wherein a is a value falling within the range of from 0.6 to 0.9.

<7> A positive electrode active material for sodium secondary batterieswhich comprises the mixed metal oxide described in any one of <1> to <6>above.

<8> A positive electrode for sodium secondary batteries which comprisesthe positive electrode active material described in <7> above.

<9> A sodium secondary battery having the positive electrode describedin <8> above.

<10> The sodium secondary battery as described in <9> above furtherhaving a separator.

<11> The sodium secondary battery as described in <10> above, whereinthe separator is a separator having a porous laminated film in which aheat-resistant porous layer comprising a heat-resistant resin and aporous film comprising a thermoplastic resin are stacked each other.

EFFECT OF THE INVENTION

According to the present invention, a sodium secondary battery capableof reducing the amount of lithium used, and ensuring a larger dischargecapacity maintenance rate when having repeated a charge and discharge ascompared with conventional techniques; and a mixed metal oxide usable asthe positive electrode active material therefor can be provided.Therefore, the present invention is very useful in industry.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A powder X-ray diffraction pattern of Mixed Metal Oxide C1.

FIG. 2 A powder X-ray diffraction pattern of Mixed Metal Oxide E1.

FIG. 3 A powder X-ray diffraction pattern of Mixed Metal Oxide E2.

FIG. 4 A powder X-ray diffraction pattern of Mixed Metal Oxide E3.

FIG. 5 A powder X-ray diffraction pattern of Mixed Metal Oxide E4.

FIG. 6 A powder X-ray diffraction pattern of Mixed Metal Oxide E7.

FIG. 7 A powder X-ray diffraction pattern of Mixed Metal Oxide E8.

FIG. 8 A powder X-ray diffraction pattern of Mixed Metal Oxide E9.

FIG. 9 A powder X-ray diffraction pattern of Mixed Metal Oxide E10.

FIG. 10 A powder X-ray diffraction pattern of Mixed Metal Oxide E11.

BEST MODE FOR CARRYING OUT THE INVENTION

<Mixed Metal Oxide of the Present Invention>

The mixed metal oxide of the present invention is characterized bycomprising Na and M¹ wherein M¹ represents three or more elementsselected from the group consisting of Mn, Fe, Co and Ni, with a Na:M¹molar ratio being a:1 wherein a is a value falling within the range ofmore than 0.5 and less than 1.

Also, the mixed metal oxide of the present invention is a mixed metaloxide represented by the following formula (1):Na_(a)M¹O₂  (1)

wherein M¹ and a each have the same meaning as above.

In one embodiment, in the mixed metal oxide of the present invention, M¹comprises at least Mn. At this time, the M¹:Mn molar ratio is preferably1:b wherein M¹ has the same meaning as above, and b is a value of notless than 0.2 and less than 1. Preferably, M¹ comprises at least Mn andNi, from the standpoint of more elevating the average discharge voltageof the obtained sodium secondary battery and increasing the energydensity of the battery. More preferably, M¹ represents Mn, Fe and Ni,that is, comprises no Co which is a scarce metal element, and even inthis case, superior effects of the present invention can be obtained. Inthe case where M¹ represents Mn, Fe and Ni, the compositional ratio (bymol) of Mn:Fe:Ni is preferably 1:(0.3 to 3):(0.01 to 2), more preferably1:(0.5 to 2):(0.1 to 1.2).

Also, in order to more enhance the effects of the present invention, ais preferably a value falling within the range of from 0.6 to 0.9, morepreferably a value falling within the range of from 0.7 to 0.9, stillmore preferably about 0.8, that is, a value falling within the range offrom 0.75 to 0.84.

In the mixed metal oxide of the present invention, a part of M¹ may besubstituted with a metal element other than M¹. By this substitution,the battery characteristics of a sodium secondary battery are enhancedin some cases.

<Production Method of Mixed Metal Oxide of the Present Invention>

The mixed metal oxide of the present invention can be produced bycalcining a metal-containing compound mixture which has a compositioncapable of giving the mixed metal oxide of the present invention aftercalcination. More specifically, metal-containing compounds comprisingcorresponding metal elements are weighed to obtain a predeterminedcomposition and mixed, and the obtained mixture is calcined, whereby themixed metal oxide can be produced. For example, a mixed metal oxidehaving a metal element ratio represented byNa:Mn:Fe:Ni=0.7:0.333:0.333:0.333 that is one of preferred metal elementratios can be produced by weighing respective raw materials of Na₂CO₃,MnO₂ and Fe₃O₄ to give an Na:Mn:Fe:Ni molar ratio of0.7:0.333:0.333:0.333, mixing these raw materials, and calcining theobtained mixture.

As the metal-containing compound usable to produce the mixed metal oxideof the present invention, an oxide, or a compound capable of becoming anoxide when decomposed and/or oxidized at a high temperature, such ashydroxide, carbonate, nitrate, halide and oxalate, can be used. Thesodium compound is preferably Na₂CO₃, NaHCO₃ or Na₂O₂, and, in view ofhandleability, more preferably Na₂CO₃. The manganese compound ispreferably MnO₂, the iron compound is preferably Fe₃O₄, the nickelcompound is preferably NiO, and the cobalt compound is preferably Co₃O₄.These metal-containing compounds may be a hydrate.

In order to mix metal-containing compounds, an apparatus usually used inindustry, such as ball mill, V-type mixer and stirrer, can be used. Themixing here may be either dry mixing or wet mixing. A mixture ofmetal-containing compounds, which has a predetermined composition, maybe obtained also by a precipitation method.

The mixture of the above-described metal-containing compounds iscalcined, for example, by keeping it at a temperature in the range offrom 600° C. to 1,600° C. over 0.5 hours to 100 hours, whereby the mixedmetal oxide of the present invention is obtained. For example, thecalcination temperature range is preferably from 600° C. to 900° C.,more preferably from 650° C. to 850° C. In the case where a compoundcapable of being decomposed and/or oxidized at a high temperature, suchas hydroxide, carbonate, nitrate, halide and oxalate, is used in themixture of metal-containing compounds, the above-described calcinationmay also be carried out after preliminarily calcining the mixture bykeeping it at a temperature in the range of from 400° C. to 1,600° C. toconvert the compound into an oxide or to remove crystal water from thecompound. The atmosphere in which the preliminary calcination is carriedout may be an inert gas atmosphere, an oxidizing atmosphere, or areducing atmosphere. After the preliminary calcination, pulverizationmay also be carried out.

The atmosphere of the calcination may be any of an inert atmosphere,such as nitrogen atmosphere and argon atmosphere; an oxidizingatmosphere, such as air atmosphere, oxygen atmosphere, oxygen-containingnitrogen atmosphere and oxygen-containing argon atmosphere; and areducing atmosphere, such as hydrogen-containing nitrogen atmospherecomprising from 0.1 vol % to 10 vol % of hydrogen, andhydrogen-containing argon atmosphere comprising from 0.1 vol % to 10 vol% of hydrogen. In order to calcine the mixture in a strongly reducingatmosphere, the calcination may be carried out after incorporating anappropriate amount of carbon into the mixture of metal-containingcompounds. Preferably, the calcination is carried out in an oxidizingatmosphere, such as air atmosphere.

By using, as the metal-containing compound, a halide such as fluoride orchloride in an appropriate amount, the crystallinity of the mixed metaloxide produced, and the average particle diameter of particlesconstituting the mixed metal oxide can be controlled. In this case, thehalide sometimes works as a reaction accelerator (flux). Examples of theflux include NaF, MnF₃, FeF₂, NiF₂, CoF₂, NaCl, MnCl₂, FeCl₂, FeCl₃,NiCl₂, CoCl₂, Na₂CO₃, NaHCO₃, NH₄Cl, NH₄I, B₂O₃ and H₃BO₃. These can beused as a raw material (metal-containing compound) of the mixture, orcan be used to be added in an appropriate amount to the mixture. Also,such a flux may be a hydrate.

In the case of using the mixed metal oxide of the present invention as apositive electrode active material for sodium secondary batteries, it issometimes preferred to adjust the particle size by optionally subjectingthe mixed metal oxide obtained as above to, for example, pulverizationby means of a ball mill, a jet mill or the like, washing andclassification. Also, calcination may be carried out two or more times.The particle of the mixed metal oxide may be surface-treated, forexample, by coating the surface of the mixed metal oxide with aninorganic substance containing Si, Al, Ti, Y or the like. The crystalstructure of the mixed metal oxide of the present invention ispreferably not a tunnel structure.

The mixed metal oxide of the present invention can be used as a positiveelectrode active material for sodium secondary batteries, as is or afterbeing subjected to a surface treatment such as coating. The positiveelectrode active material comprises the mixed metal oxide of the presentinvention. When the mixed metal oxide of the present invention is usedfor a sodium secondary battery, the obtained secondary battery can havea larger discharge capacity maintenance rate when having repeated acharge and discharge, as compared with conventional techniques. Also, bythe present invention, the internal resistance of the obtained sodiumsecondary battery can be made small, and the overvoltage during chargingand discharging can be reduced. Due to reduction in the overvoltageduring charging and discharging, the large current discharge property ofthe secondary battery can be enhanced. Also, the stability of thebattery in the case of the overcharging the secondary battery can beenhanced.

<Positive Electrode for Sodium Secondary Batteries of the PresentInvention and Production Method Thereof>

A positive electrode for sodium secondary batteries of the presentinvention comprises the positive electrode active material of thepresent invention. The positive electrode for sodium secondary batteriesof the present invention can be produced by loading, on a positiveelectrode current collector, a positive electrode mixture comprising thepositive electrode active material of the present invention, anelectrically conductive material and a binder.

Examples of the electrically conductive material include a carbonaceousmaterial, such as natural graphite, artificial graphite, coke, andcarbon black. Examples of the binder include a thermoplastic resin, andspecific examples thereof include a fluororesin, such as polyvinylidenefluoride (hereinafter referred to as “PVDF”), polytetrafluoroethylene,ethylene tetrafluoride-propylene hexafluoride-vinylidene fluoride-basedcopolymer, propylene hexafluoride-vinylidene fluoride-based copolymer,and ethylene tetrafluoride-perfluorovinyl ether-based copolymer; and apolyolefin resin, such as polyethylene and polypropylene. Examples ofthe positive electrode current collector include Al, Ni and stainlesssteel.

The method for loading a positive electrode mixture on, a positiveelectrode current collector includes a method of pressure-molding themixture, and a method of forming the positive electrode mixture into apaste by using an organic solvent or the like, applying and drying thepaste on a positive electrode current collector, and fixing the mixtureby pressing or the like. In the case of forming a paste, a slurrycomprising a positive electrode active material, an electricallyconductive material, a binder and an organic solvent is prepared.Examples of the organic solvent include an amine-based, solvent, such asN,N-dimethylaminopropylamine and diethyltriamine; an ether-basedsolvent, such as ethylene oxide and tetrahydrofuran; a ketone-basedsolvent, such as methyl ethyl ketone; an ester-based solvent, such asmethyl acetate; and an aprotic polar solvent, such as dimethylacetamideand N-methyl-2-pyrrolidone. Examples of the method for applying apositive electrode mixture on a positive electrode current collectorinclude a slit die coating method, a screen coating method, a curtaincoating method, a knife coating method, a gravure coating method, and anelectrostatic spraying method.

<Sodium Secondary Battery of the Present Invention>

A sodium secondary battery of the present invention has the positiveelectrode for sodium secondary batteries of the present invention. Thesodium secondary battery of the present invention can be produced, forexample, by stacking the positive electrode for sodium secondarybatteries of the present invention, a separator, and a negativeelectrode comprising a negative electrode current collector havingloaded thereon a negative electrode mixture, in this order; winding thestack to yield an electrode group; housing the electrode group in abattery can; and then impregnating the electrode group with anelectrolytic solution composed of an organic solvent comprising anelectrolyte.

Examples of the shape of the electrode group include a shape that givesa cross section of a circular shape, an elliptical shape, an oval shape,a rectangular shape, a corner-rounded rectangular shape or the like,when the electrode group is cut in the direction perpendicular to thewinding axis. Examples of the shape of the battery include a papershape, a coin shape, a cylinder shape, and a square shape.

<Sodium Secondary Battery of the Present Invention/Negative Electrode>

A negative electrodes usable in the sodium secondary battery of thepresent invention include an electrode capable of storing and releasingsodium ions such as a member obtained by loading, on a negativeelectrode current collector, a negative electrode mixture comprising anegative electrode active material, sodium metal and a sodium alloy. Thenegative electrode active material includes a carbonaceous materialcapable of storing and releasing sodium ions, such as natural graphite,artificial graphite, coke, carbon black, pyrolytic carbons, carbon fiberand calcined organic polymer compound. The shape of the carbonaceousmaterial may be any of a flake, such as that of natural graphite, asphere, such as that of mesocarbon microbead, a fiber, such as that ofgraphitized carbon fiber, or an aggregate of fine powder. Thecarbonaceous material may also work as an electrically conductivematerial.

As for the negative electrode active material, a chalcogen compound,such as oxide and sulfide, capable of storing and releasing sodium ionsat a lower potential than a positive electrode may also be used.

The negative electrode mixture may comprise a binder and an electricallyconductive material, if necessary. Accordingly, the negative electrodeof the sodium secondary battery of the present invention may beconfigured to comprise a mixture of a carbonaceous material and abinder. The binder includes a thermoplastic resin, and specific examplesthereof include PVDF, thermoplastic polyimide, carboxymethyl cellulose,polyethylene and polypropylene.

Examples of the negative electrode current collector include Cu, Ni andstainless steel, and Cu is preferred because Cu is difficult to be analloy with sodium, and is easily formed into a thin film. Examples ofthe method for loading a negative electrode mixture on a negativeelectrode current collector are the same as in the case of a positiveelectrode, and include a method of pressure-molding the mixture, and amethod of forming the negative electrode mixture into a paste by using asolvent or the like, applying and drying the paste on a negativeelectrode current collector, and fixing the mixture by pressing or thelike.

<Sodium Secondary Battery of the Present Invention/Separator>

As a separator usable in the sodium secondary battery of the presentinvention, a member having a form, such as porous film, nonwoven fabricand woven fabric, and made of a material of a polyolefin resin, such aspolyethylene and polypropylene, a fluororesin or a nitrogen-containingaromatic polymer can be used. A single-layer or multilayer separatorusing two or more of these materials may also be used. Examples of theseparator include separators described in Japanese Unexamined PatentPublication Nos. 2000-30686 and 10-324758 . A thickness of the separatoris preferably smaller as long as the mechanical strength can bemaintained, from the standpoint of increase in the volumetric energydensity of a battery and decrease in internal resistance thereof. Ingeneral, a thickness of the separator is preferably about 5 to 200 μm,more preferably about 5 to 40 μm.

The separator preferably has a porous film comprising a thermoplasticresin. In a secondary battery, the separator is located between apositive electrode and a negative electrode. When an extraordinarycurrent flows in the battery due to short-circuit between a positiveelectrode and a negative electrode, or the like, the separatorpreferably plays a role by which the current is blocked to prevent anovercurrent from flowing (to shutdown). The shutdown is achieved byshutting fine pores of the porous film of the separator when thetemperature exceeds a usual use temperature. Even when the temperaturein the battery rises to a certain high temperature after the shutdown,it is preferable that the separator maintain the shutdown state withoutbeing ruptured due to the temperature, in other words, have high heatresistance. This separator includes a porous film having aheat-resistant material such as a porous laminated film in which aheat-resistant porous layer and a porous film are stacked each other,preferably a porous laminated film in which a heat-resistant porouslayer comprising a heat-resistant resin and a porous film comprising athermoplastic resin are stacked each other. By using such a porous filmhaving a heat-resistant material as a separator, the secondary batteryof the present invention can be more successfully prevented from thermalfilm rupture. The heat-resistant porous layer can be stacked on bothsides of the porous film.

<Sodium Secondary Battery of the Present Invention/Separator/PorousLaminate Film Separator>

The separator composed of a porous laminated film in which aheat-resistant porous layer and a porous film are stacked each other isdescribed below. A thickness of the separator is usually from 5 μm to 40μm, preferably 20 μm or less. Assuming that a thickness of theheat-resistant porous layer is A (μm) and a thickness of the porous filmis B (μm), the value of A/B is preferably from 0.1 to 1. Considering theion permeability, the permeability of the separator is, in terms ofGurley permeability, preferably from 50 to 300 seconds/100 ml, morepreferably from 50 to 200 seconds/100 ml. A void content of theseparator is usually from 30 to 80 vol %, and preferably from 40 to 70vol %.

(Heat-Resistant Porous Layer)

In the porous laminated film, the heat-resistant porous layer preferablycomprises a heat-resistant resin. In order to elevate the ionpermeability, a thickness of the heat-resistant porous layer ispreferably from 1 μm to 10 μm, more preferably from 1 μm to 5 μm, andparticularly preferably from 1 μm to 4 μm to be a thinner heat-resistantporous layer. The heat-resistant porous layer has fine pores, and thesize (diameter) of the pore is usually 3 μm or less, preferably 1 μm orless. The heat-resistant porous layer may comprise a filler describedlater. The heat-resistant porous layer may be formed from an inorganicpowder.

The heat-resistant resin contained in the heat-resistant porous layerincludes polyamide, polyimide, polyamideimide, polycarbonate,polyacetal, polysulfone, polyphenylene sulfide, polyether ketone,aromatic polyester, polyethersulfone and polyetherimide. From thestandpoint of further enhancing the heat resistance, polyamide,polyimide, polyamideimide, polyethersulfone and polyetherimide arepreferred; and polyamide, polyimide and polyamideimide are morepreferred. The heat-resistant resin is more preferably anitrogen-containing aromatic polymer, such as aromatic polyamide(para-oriented aromatic polyamide, meta-oriented aromatic polyamide),aromatic polyimide and aromatic polyamideimide, still more preferably anaromatic polyamide, and yet still more preferably a para-orientedaromatic polyamide (hereinafter, referred to as “para-aramide”). Inaddition, the heat-resistant resin also includes poly-4-methylpentene-1,and a cyclic olefin-based polymer. By using such a heat-resistant resin,the heat resistance can be enhanced, i.e. the thermal film rupturetemperature can be raised.

The thermal film rupture temperature depends on the types ofheat-resistant resin, and is selected and used on the basis of thesituation and the purpose of the use thereof. The thermal film rupturetemperature is usually 160° C. or more. The thermal film rupturetemperature can be controlled to about 400° C. in the case of using theabove-described nitrogen-containing aromatic polymer, to about 250° C.in the case of using poly-4-methylpentene-1, and to about 300° C. in thecase of using a cyclic olefin-based polymer, as the heat-resistantresin, respectively. The thermal film rupture temperature can becontrolled to, for example, 500° C. or more in the case of using aheat-resistant porous layer formed from an inorganic powder.

The para-aramide is obtained by condensation polymerization of apara-oriented aromatic diamine and a para-oriented aromatic dicarboxylicacid halide, and is substantially composed of a repeating unit where theamide bond is bonded at the pars-position or equivalently orientedposition of the aromatic ring (for example, the oriented positionextending coaxially or in parallel to the opposite direction, such as4,4′-biphenylene, 1,5-naphthalene, and 2,6-naphthalene). Thepara-aramide includes a para-aramide having a para-oriented-type andquasi-para-oriented-type structures. Specific examples thereof includepoly(paraphenyleneterephthalamide), poly(parabenzamide),poly(4,4′-benzanilideterephthalamide),poly(paraphenylene-4,4′-biphenylenedicarboxylic acid amide),poly(paraphenylene-2,6-naphthalenedicarboxylic acid amide),poly(2-chloroparaphenyleneterephthalamide), andparaphenyleneterephthalamide/2,6-dichloroparaphenyleneterephthalamidecopolymer.

The aromatic polyimide is preferably a wholly aromatic polyimideproduced by condensation polymerization of an aromatic diacid anhydrideand an aromatic diamine. Specific examples of the diacid anhydrideinclude pyromellitic dianhydride,3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride,3,3′,4,4′-benzophenonetetracarboxylic dianhydride,2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane, and3,3′,4,4′-biphenyltetracarboxylic dianhydride. Examples of the diamineinclude oxydianiline, para-phenylenediamine, benzophenonediamine,3,3′-methylenedianiline, 3,3′-diaminobenzophenone,3,3′-diaminodiphenylsulfone, and 1,5′-naphthalenediamine. A polyimidesoluble in a solvent may be suitably used. Examples of such a polyimideinclude a polyimide as a polycondensate of3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride with an aromaticdiamine.

Examples of the aromatic polyamideimide include those obtained bycondensation polymerization of an aromatic dicarboxylic acid and anaromatic diisocyanate, and those obtained by condensation polymerizationof an aromatic diacid anhydride and an aromatic diisocyanate. Specificexamples of the aromatic dicarboxylic acid include isophthalic acid andterephthalic acid. Specific examples of the aromatic diacid anhydrideinclude trimellitic anhydride. Specific examples of the aromaticdiisocyanate include 4,4′-diphenylmethane diisocyanate, 2,4-tolylenediisocyanate, 2,6-tolylene diisocyanate, ortho-tolylene diisocyanate andm-xylylene diisocyanate.

When the heat-resistant porous layer comprises a heat-resistant resin,the heat-resistant porous layer may comprise one or more types offillers. The filler that may be contained in the heat-resistant porouslayer may be any one selected from an organic powder, an inorganicpowder and a mixture thereof. The average particle diameter of theparticle constituting the filler is preferably from 0.01 μm to 1 μm.Examples of the shape of the filler include an approximately sphericalshape, a plate shape, a columnar shape, an acicular particle, a whiskershape and a fibrous shape, and any particles of these shapes may beused. The filler is preferably an approximately spherical particle dueto ease in forming uniform pores. The approximately spherical particlesinclude particles having an aspect ratio (longer diameter ofparticle/shorter diameter of particle) in the range of from 1 to 1.5 .The aspect ratio of particles can be determined using an electronmicroscope.

The organic powder as the filler includes a powder composed of anorganic material, such as a homopolymer of or a copolymer of two or moreof styrene, vinyl ketone, acrylonitrile, methyl methacrylate, ethylmethacrylate, glycidyl methacrylate, glycidyl acrylate and methylacrylate; a fluororesin, such as polytetrafluoroethylene,tetrafluoroethylene-hexafluoropropylene copolymer,tetrafluoroethylene-ethylene copolymer and polyvinylidene fluoride; amelamine resin; a urea resin; a polyolefin; and polymethacrylate. Theorganic powders may be used solely, or in admixture of two or more.Among the organic powders, a polytetrafluoroethylene powder is preferredin view of chemical stability.

Examples of the inorganic powder as the filler include a powder composedof an inorganic material, such as metal oxide, metal nitride, metalcarbide, metal hydroxide, carbonate and sulfate. Among those, a powdercomposed of an inorganic material having a small conductivity can bepreferably used. Specific examples thereof include a powder composed ofalumina, silica, titanium dioxide, barium sulfate, or calcium carbonate.The inorganic powders may be used solely, or in admixture of two ormore. Among the inorganic powders, an alumina powder is preferred inview of chemical stability. It is preferred that all of the particlesconstituting the filler be an alumina particle, and more preferred thatall of the particles constituting the filler be an alumina particle, anda part or all thereof are an approximately spherical alumina particle.Incidentally, when the heat-resistant porous layer is formed from aninorganic powder, the above inorganic powder can be used, optionallyalong with a binder if required.

When the heat-resistant porous layer comprises a heat-resistant resin,the content of the filler in the heat-resistant porous layer variesdepending on the specific gravity of the material of the filler. Forexample, in the case where all of the particles constituting the fillerare alumina particles, the weight of the filler is usually from 5 to 95parts by weight, preferably from 20 to 95 parts by weight, and morepreferably from 30 to 90 parts by weight, assuming that the total weightof the heat-resistant porous layer is 100 parts by weight. These rangescan be appropriately set, depending on the specific gravity of thematerial of the filler.

(Porous Film)

In the porous laminated film, the porous film. preferably has finepores, and can shutdown. In this case, the porous film comprises athermoplastic resin. A thickness of the porous film is usually from 3 to30 μm, preferably from 3 to 25 μm. The porous film has fine poressimilarly to the heat-resistant porous layer, and the size of the poreis usually 3 μm or less, preferably 1 μm or less. A void content of theporous film is usually from 30 to 80 vol %, preferably from 40 to 70 vol%. When a temperature of nonaqueous electrolyte secondary batteryexceeds a usual use temperature, the porous film can shut the fine poresdue to softening of the thermoplastic resin constituting the porousfilm.

The thermoplastic resin contained in the porous film includes a resinthat is softened at from 80 to 180° C., and a thermoplastic resin whichdoes not dissolve in the electrolytic solution of a nonaqueouselectrolyte secondary battery may be selected. Specific examples of thethermoplastic resin include a polyolefin resin, such as polyethylene andpolypropylene, and a thermoplastic polyurethane resin. A mixture of twoor more of these resins may be used. In order to perform a shutdown bysoftening at a lower temperature, the thermoplastic resin preferablycomprises polyethylene. The polyethylene specifically includes apolyethylene, such as a low-density polyethylene, a high-densitypolyethylene and a linear polyethylene, and also includes an ultrahighmolecular-weight polyethylene having a molecular weight of one millionor more. For further enhancing the piercing strength of the porous film,the thermoplastic resin preferably comprises at least an ultrahighmolecular-weight polyethylene. In view of production of the porous film,it is sometimes preferred that the thermoplastic resin comprise a waxcomposed of a polyolefin of low molecular-weight (weight averagemolecular weight of 10,000 or less).

The examples of a porous film comprising a heat-resistant material whichdiffers from that of the above-described porous laminated film include aporous film formed from a heat-resistant resin and/or an inorganicpowder, and a porous film in which a heat-resistant resin and/or aninorganic powder is dispersed in a thermoplastic resin film ofpolyolefin resin, thermoplastic polyurethane resin or the like. Theheat-resistant resin and the inorganic powder include ones describedabove.

<Sodium Secondary Battery of the Present Invention/Electrolytic Solutionor Solid Electrolyte>

In the electrolytic solution usable in the sodium secondary battery ofthe present invention, examples of the electrolyte include NaClO₄,NaPF₆, NaAsF₆, NaSbF₆, NaBF₄, NaCF₃SO₃, NaN(SO₂CF₃)₂, sodium salt oflower aliphatic carboxylate, and NaAlCl₄. A mixture of two or morethereof may be used. Among these, an electrolyte comprising, at leastone selected from the group consisting of NaPF₆, NaAsF₆, NaSbF₆, NaBF₄,NaCF₃SO₃ and NaN(SO₂CF₃)₂, which comprise fluorine, is preferably used.

In the electrolytic solution usable in the sodium secondary battery ofthe present invention, examples of the organic solvent, which can beused, include carbonates, such as propylene carbonate, ethylenecarbonate, dimethyl carbonate, diethyl carbonate, ethyl methylcarbonate, isopropyl methyl carbonate, vinylene carbonate,4-trifluoromethyl-1,3-dioxolan-2-one, and1,2-di(methoxycarbonyloxy)ethane; ethers, such as 1,2-dimethoxyethane,1,3-dimethoxypropane, pentafluoropropylmethyl ether,2,2,3,3-tetrafluoropropyldifluoromethyl ether, tetrahydrofuran, and2-methyltetrahydrofuran; esters, such as methyl formate, methyl acetate,and γ-butyrolactone; nitrites, such as acetonitrile and butyronitrile;amides, such as N,N-dimethylformamide, and N,N-dimethylacetamide;carbamates, such as 3-methyl-2-oxazolidone; sulfur-containing compounds,such as sulfolane, dimethyl sulfoxide, and 1,3-propanesultone; and thoseobtained by introducing a fluorine substituent into the organic solventabove. Usually, two or more of the organic solvents are mixed and used.

A solid electrolyte may also be used in place of the electrolyticsolution. Examples of the solid electrolyte which can be used include anorganic solid electrolyte, such as polyethylene oxide-based polymer, andpolymer comprising at least one or more of polyorganosiloxane chains orpolyoxyalkylene chains. A so-called gel-type electrolyte holding anonaqueous electrolyte solution in a polymer can also be used. An inorganic solid electrolyte such as Na₂S—SiS₂, Na₂S—GeS₂, NaTi₂(PO₄)₃,NaFe₂(PO₄)₃, Na₂(SO₄)₃, Fe₂(SO₄)₂(PO₄) and Fe₂(MoO₄)₃ may also be used.When such a solid electrolyte is used, safety can be enhanced in somecases. In the case of using a solid electrolyte in the sodium secondarybattery of the present invention, the solid electrolyte sometimes worksas a separator, and in this case, a separator may not be necessary.

EXAMPLES

The present invention is described in greater detail below by referringto examples, but the present invention is not limited thereto by anymeans. Incidentally, unless otherwise indicated, a production method ofan electrode and a secondary battery for a. charge/discharge test, and ameasurement method of powder X-ray diffraction are as follows.

(1) Production of Electrode (Positive Electrode)

A mixed metal oxide as a positive electrode active material, anacetylene black (produced by Denki Kagaku Kogyo Kabushiki Kaisha) as anelectrically conductive material, and PVDF (PolyVinylidine DiFluoridePolyflon, produced by Kureha Corporation) as a binder were weighed sothat a composition of positive electrode active material:electricallyconductive material:binder may be 85:10:5 (by weight). Thereafter, themixed metal oxide and the acetylene black were thoroughly mixed in anagate mortar, an appropriate amount of N-methyl-2-pyrrolidone (NMP,produced by Tokyo Chemical Industry Co., Ltd.) was added to the mixture,PVDF was further added thereto, and these were then uniformly mixed toform a slurry. The obtained slurry was applied on a 40 μm-thick aluminumfoil as a current collector by using an applicator to a thickness of 100μm of the slurry, and the aluminum foil having the applied slurry wasplaced in a drier and thoroughly dried by removing NMP to yield anelectrode sheet. This electrode sheet was punched with a diameter of 1.5cm by an electrode punch, and sufficiently fixed under pressure by ahand press to yield a positive electrode sheet.

(2) Production of Battery

The positive electrode sheet was placed in a recess of the bottom partof a coin cell (manufactured by Hohsen Corp.) by arranging the aluminumfoil to face downward, and combined with a 1 M NaClO₄/propylenecarbonate as an electrolyte, a polypropylene porous film (thickness: 20μm) as a separator, and a sodium metal (produced by Aldrich ChemicalCompany, Inc.) as a negative electrode to produce a battery. Assemblingof the battery was performed in a glove box under an argon atmosphere.

(3) Powder X-Ray Diffraction Measurement

The measurement was performed under the following conditions by using apowder X-ray diffraction measuring apparatus, Model RINT2500TTR,manufactured by Rigaku Corporation.

X-ray: CuKα

Voltage-current: 40 kV-140 mA

Measuring angle range: 2θ=10-60°

Step: 0.02°

Scan speed: 4°/min

Comparative Example 1 (Na:Mn:Co=0.7:0.50:0.50)

(1) Production of Mixed Metal Oxide

Sodium carbonate (Na₂CO₃, produced by Wako Pure Chemical Industries,Ltd., purity: 99.8%), manganese(IV) oxide (MnO₂, produced by KojundoChemical Laboratory Co., Ltd., purity: 99.9%) and tricobalt tetraoxide(Co₃O₄, produced by Seido Chemical Industry Co., Ltd. purity: 99%), asmetal-containing compounds, were weighed to obtain an Na:Mn:Co molarratio of 0.7:0.50:0.50, and mixed in a dry ball mill over 4 hours toobtain a mixture of the metal-containing compounds. The obtained mixtureof the metal-containing compounds was filled in an alumina boat, heatedin an air atmosphere by using an electric furnace, and kept at 800° C.over 2 hours to obtain Mixed Metal Oxide C1 of Comparative Example 1.FIG. 1 shows the powder X-ray diffraction pattern of Mixed Metal OxideC1 of Comparative Example 1.

(2) Evaluation of Charge/Discharge Performance as Positive ElectrodeActive Material of Sodium Secondary Battery

A battery was produced by using Mixed Metal Oxide C1 of ComparativeExample 1 as the positive electrode active material for sodium secondarybatteries, and subjected to a constant current charge/discharge testunder the following conditions.

Charge/Discharge Conditions:

The charge was performed by CC (constant current) charge at a 0.1 C rate(a rate that requires 10 hours for full charge) up to 4.0 V. Thedischarge was performed by CC discharge at the same rate as the chargerate, and the current was cut off at a voltage of 1.5 V.Charge/discharge for the next and subsequent cycles was performed at thesame rate as the charge rate above, and the current was cut off at acharge voltage of 4.0 V and a discharge voltage of 1.5 V similarly tothe 1st cycle.

Charge/discharge of this battery was repeated 10 cycles, and thedischarge capacity maintenance rate for 10th cycle was 76% based on thedischarge capacity for 1st cycle.

Example 1 (Na:Mn:Co:Ni=0.7:0.495:0.495:0.01; in Formula (1), M¹ is Mn,Co and Ni, a is 0.7, and b is 0.495)

(1) Production of Mixed Metal Oxide

Sodium carbonate (Na₂CO₃, produced by Wako Pure Chemical Industries,Ltd., purity: 99.8%), manganese(IV) oxide (MnO₂, produced by KojundoChemical Laboratory Co., Ltd., purity: 99.9%), tricobalt tetraoxide(Co₃O₄, produced by Seido Chemical Industry Co., Ltd. purity: 99%) andnickel(II) oxide (NiO, produced by Kojundo Chemical Laboratory Co.,Ltd., purity: 99%), as metal-containing compounds, were weighed toobtain an Na:Mn:Co:Ni molar ratio of 0.7:0.495:0.495:0.01, and mixed ina dry ball mill over 4 hours to obtain a mixture of the metal-containingcompounds. The obtained mixture of the metal-containing compounds wasfilled in an alumina boat, heated in an air atmosphere by using anelectric furnace, and kept at 800° C. over 2 hours to obtain Mixed MetalOxide E1 of Example 1. FIG. 2 shows the powder X-ray diffraction patternof Mixed Metal Oxide E1 of Example 1.

(2) Evaluation of Charge/Discharge Performance as Positive ElectrodeActive Material of Sodium Secondary Battery

A battery was produced by using Mixed Metal Oxide E1 of Example 1 as thepositive electrode active material for sodium secondary batteries, andsubjected to a constant current charge/discharge test under the sameconditions as in Comparative Example 1. Charge/discharge of this batterywas repeated 10 cycles, and the discharge capacity maintenance rate for10th cycle was 109% based on the discharge capacity for 1st cycle.

Example 2 (Na:Mn:Co:Ni=0.7:0.45:0.45:0.10; in Formula (1), M¹ is Mn, Coand Ni, a is 0.7, and b is 0.45)

(1) Production of Mixed Metal Oxide

Mixed Metal Oxide E2 of Example 2 was obtained in the same manner as inExample 1 except for using the metal-containing compounds in amounts ofobtaining an Na:Mn:Co:Ni molar ratio of 0.7:0.45:0.45:0.10 . FIG. 3shows the powder X-ray diffraction pattern of Mixed Metal Oxide E2 ofExample 2.

(2) Evaluation of Charge/Discharge Performance as Positive ElectrodeActive Material of Sodium Secondary Battery

A battery was produced by using Mixed Metal Oxide E2 of Example 2 as thepositive electrode active material for sodium secondary batteries, andsubjected to a constant current charge/discharge test under the sameconditions as in Comparative Example 1. Charge/discharge of this batterywas repeated 10 cycles, and the discharge capacity maintenance rate for10th cycle was 89% based on the discharge capacity for 1st cycle.

Example 3 (Na:Mn:Co:Ni=0.7:0.333:0.333:0.333; in Formula (1), M¹ is Mn,Co and Ni, a is 0.7, and b is 0.333)

(1) Production of Mixed Metal Oxide

Mixed Metal Oxide E3 of Example 3 was obtained in the same manner as inExample 1 except for using the metal-containing compounds in amounts ofobtaining an Na:Mn:Co:Ni molar ratio of 0.7:0.333:0.333:0.333 . FIG. 4shows the powder x-ray diffraction pattern of Mixed Metal Oxide E3 ofExample 3.

(2) Evaluation of Charge/Discharge Performance as Positive ElectrodeActive Material of Sodium Secondary Battery

A battery was produced by using Mixed Metal Oxide E3 of Example 3 as thepositive electrode active material for sodium secondary batteries, andsubjected to a constant current charge/discharge test under the sameconditions as in Comparative Example 1. Charge/discharge of this batterywas repeated 10 cycles, and the discharge capacity maintenance rate for10th cycle was 90% based on the discharge capacity for 1st cycle.

Example 4 (Na:Mn:Fe:Ni=0.7:0.333:0.333:0.333; in Formula (1), M¹ is Mn,Fe and Ni, a is 0.7, and b is 0.333)

(1) Production of Mixed Metal Oxide

Sodium carbonate (Na₂CO₃, produced by Wako Pure Chemical Industries,Ltd., purity: 99.8%), manganese(IV) oxide (MnO₂, produced by KojundoChemical Laboratory Co., Ltd., purity: 99.9%), iron(II, III) oxide(Fe₃O₄, produced by Kojundo Chemical Laboratory Co., Ltd., purity: 99%)and nickel(II) oxide (NiO, produced by Kojundo Chemical Laboratory Co.,Ltd., purity: 99%), as metal-containing compounds, were weighed toobtain an Na:Mn:Fe:Ni molar ratio of 0.7:0.333:0.333:0.333, and mixed ina dry ball mill over 4 hours to obtain a mixture of the metal-containingcompounds. The obtained mixture of the metal-containing compounds wasfilled in an alumina boat, heated in an air atmosphere by using anelectric furnace, and kept at 800° C. over 2 hours to obtain Mixed MetalOxide E4 of Example 4. FIG. 5 shows the powder X-ray diffraction patternof Mixed Metal Oxide E4 of Example 4.

(2) Evaluation of Charge/Discharge Performance as Positive ElectrodeActive Material of Sodium Secondary Battery

A battery was produced by using Mixed Metal Oxide E4 of Example 4 as thepositive electrode active material for sodium secondary batteries, andsubjected to a constant current charge/discharge test under the sameconditions as in Comparative Example 1. Charge/discharge of this batterywas repeated 10 cycles, and the discharge capacity maintenance rate for10th cycle was 83% based on the discharge capacity for 1st cycle.

Example 5 (Na:Mn:Fe:Co=0.7:0.333:0.333:0.333; in Formula (1), M¹ is Mn,Fe and Co, a is 0.7, and b is 0.333)

(1) Production of Mixed Metal Oxide

Sodium carbonate (Na₂CO₃, produced by Wako Pure Chemical Industries,Ltd., purity: 99.8%), manganese(IV) oxide (MnO₂, produced by KojundoChemical Laboratory Co., Ltd., purity: 99.9%), iron(II, III) oxide(Fe₃O₄, produced by Kojundo Chemical Laboratory Co., Ltd., purity: 99%)and tricobalt tetroxide (Co₃O₄, produced by Seido Chemical Industry Co.,Ltd. purity: 99%), as metal-containing compounds, were weighed to obtainan Na:Mn:Fe:Co molar ratio of 0.7:0.333:0.333:0.333, and mixed in a dryball mill over 4 hours to obtain a mixture of the metal-containingcompounds. The obtained mixture of the metal-containing compounds wasfilled in an alumina boat, heated in an air atmosphere by using anelectric furnace, and kept at 800° C. over 2 hours to obtain Mixed MetalOxide E5 of Example 5.

(2) Evaluation of Charge/Discharge Performance as Positive ElectrodeActive Material of Sodium Secondary Battery

A battery was produced by using Mixed Metal Oxide E5 of Example 5 as thepositive electrode active material for sodium secondary batteries, andsubjected to a constant current charge/discharge test under the sameconditions as in Comparative Example 1 . Charge/discharge of thisbattery was repeated 10 cycles, and the discharge capacity maintenancerate for 10th cycle was 78% based on the discharge capacity for 1stcycle.

Example 6 (Na:Mn:Fe:Co:Ni=0.7:0.25:0.25:0.25:0.25; in Formula (1), M¹ isMn, Fe, Co and Ni, a is 0.7, and b is 0.25)

(1) Production of Mixed Metal Oxide

Sodium carbonate (Na₂CO₃, produced by Wako Pure Chemical Industries,Ltd., purity: 99.8%), manganese(IV) oxide (MnO₂, produced by KojundoChemical Laboratory Co., Ltd., purity: 99.9%), iron(II, III) oxide(Fe₃O₄, produced by Kojundo Chemical Laboratory Co., Ltd., purity: 99%),tricobalt tetroxide (Co₃O₄, produced by Seido Chemical Industry Co.,Ltd. purity: 99%) and nickel(II) oxide (NiO, produced by KojundoChemical Laboratory Co., Ltd., purity: 99%), as metal-containingcompounds, were weighed to obtain an Na:Mn:Fe:Co:Ni molar ratio of0.7:0.25:0.25:0.25:0.25, and mixed in a dry ball mill over 4 hours toobtain a mixture of the metal-containing compounds. The obtained mixtureof the metal-containing compounds was filled in an alumina boat, heatedin an air atmosphere by using an electric furnace, and kept at 800° C.over 2 hours to obtain Mixed Metal Oxide E6 of Example 6.

(2) Evaluation of Charge/Discharge Performance as Positive ElectrodeActive Material of Sodium Secondary Battery

A battery was produced by using Mixed Metal Oxide E6 of Example 6 as thepositive electrode active material for sodium secondary batteries, andsubjected to a constant current charge/discharge test under the sameconditions as in Comparative Example 1 . Charge/discharge of thisbattery was repeated 10 cycles, and the discharge capacity maintenancerate for 10th cycle was 88% based on the discharge capacity for 1stcycle.

Example 7 (Na:Mn:Fe:Ni=0.8:0.333:0.333:0.333; in Formula (1), M¹ is Mn,Fe and Ni, a is 0.8, and b is 0.333)

(1) Production of Mixed Metal Oxide

Sodium carbonate (Na₂CO₃, produced by Wako Pure Chemical Industries,Ltd., purity: 99.8%), manganese(IV) oxide (MnO₂, produced by KojundoChemical Laboratory Co., Ltd., purity: 99.9%), iron(II, III) oxide(Fe₃O₄, produced by Kojundo Chemical Laboratory Co., Ltd., purity: 99%)and nickel(II) oxide (NiO, produced by Kojundo Chemical Laboratory Co.,Ltd., purity: 99%), as metal-containing compounds, were weighed toobtain an Na:Mn:Fe:Ni molar ratio of 0.8:0.333:0.333:0.333, and mixed ina dry ball mill over 4 hours to obtain a mixture of the metal-containingcompounds. The obtained mixture of the metal-containing compounds wasfilled in an alumina boat, heated in an air atmosphere by using anelectric furnace, and kept at 800° C. over 2 hours to obtain Mixed MetalOxide E7 of Example 7. FIG. 6 shows the powder X-ray diffraction patternof Mixed Metal Oxide E7 of Example 7.

(2) Evaluation of Charge/Discharge Performance as Positive ElectrodeActive Material of Sodium Secondary Battery

A battery was produced by using Mixed Metal Oxide E7 of Example 7 as thepositive electrode active material for sodium secondary batteries, andsubjected to a constant current charge/discharge test under the sameconditions as in Comparative Example 1 . Charge/discharge of thisbattery was repeated 10 cycles, and the discharge capacity maintenancerate for 10th cycle was 93% based on the discharge capacity for 1stcycle.

Example 8 (Na:Mn:Fe:Ni=0.9:0.333:0.333:0.333; in Formula (1), M¹ is Mn,Fe and Ni, a is 0.9, and b is 0.333)

(1) Production of Mixed Metal Oxide

Sodium carbonate (Na₂CO₃, produced by Wako Pure Chemical Industries,Ltd., purity: 99.8%), manganese(IV) oxide (MnO₂, produced by KojundoChemical Laboratory Co., Ltd., purity: 99.9%), iron(II, III) oxide(Fe₃O₄, produced by Kojundo Chemical Laboratory Co., Ltd., purity: 99%)and nickel(II) oxide (NiO, produced by Kojundo Chemical Laboratory Co.,Ltd., purity: 99%), as metal-containing compounds, were weighed toobtain an Na:Mn:Fe:Ni molar ratio of 0.9:0.333:0.333:0.333, and mixed ina dry ball mill over 4 hours to obtain a mixture of the metal-containingcompounds. The obtained mixture of the metal-containing compounds wasfilled in an alumina boat, heated in an air atmosphere by using anelectric furnace, and kept at 800° C. over 2 hours to obtain Mixed MetalOxide E8 of Example 8. FIG. 7 shows the powder X-ray diffraction patternof the Mixed Metal Oxice E8 of Example 8.

(2) Evaluation of Charge/Discharge Performance as Positive ElectrodeActive Material of Sodium Secondary Battery

A battery was produced by using Mixed Metal Oxide E8 of Example 8 as thepositive electrode active material for sodium secondary batteries, andsubjected to a constant current charge/discharge test under the sameconditions as in Comparative Example 1 . Charge/discharge of thisbattery was repeated 10 cycles, and the discharge capacity maintenancerate for 10th cycle was 88% based on the discharge capacity for 1stcycle.

Example 9 (Na:Mn:Fe:Ni=0.7:0.38:0.38:0.24; in Formula (1), M¹ is Mn, Feand Ni, a is 0.7, and b is 0.38)

(1) Production of Mixed Metal Oxide

Sodium carbonate (Na₂CO₃, produced by Wako Pure Chemical Industries,Ltd., purity: 99.8%), manganese(IV) oxide (MnO₂, produced by KojundoChemical Laboratory Co., Ltd., purity: 99.9%), iron(II, III) oxide(Fe₃O₄, produced by Kojundo Chemical Laboratory Co., Ltd., purity: 99%)and nickel(II) oxide (NiO, produced by Kojundo Chemical Laboratory Co.,Ltd., purity: 99%), as metal-containing compounds, were weighed toobtain an Na:Mn:Fe:Ni molar ratio of 0.7:0.38:0.38:0.24, and mixed in adry ball mill over 4 hours to obtain a mixture of the metal-containingcompounds. The obtained mixture of the metal-containing compounds wasfilled in an alumina boat, heated in an air atmosphere by using anelectric furnace, and kept at 800° C. over 2 hours to obtain Mixed MetalOxide E9 of Example 9. FIG. 8 shows the powder X-ray diffraction patternof Mixed Metal Oxide E9 of Example 9.

(2) Evaluation of Charge/Discharge Performance as Positive ElectrodeActive Material of Sodium Secondary Battery

A battery was produced by using Mixed Metal Oxide E9 of Example 9 as thepositive electrode active material for sodium secondary batteries, andsubjected to a constant current charge/discharge test under the sameconditions as in. Comparative Example 1. Charge/discharge of thisbattery was repeated 10 cycles, and the discharge capacity maintenancerate for 10th cycle was 93% based on the discharge capacity for 1stcycle.

Example 10 (Na:Mn:Fe:Ni=0.7:0.42:0.42:0.16; in Formula (1), M¹ is Mn, Feand Ni, a is 0.7, and b is 0.42)

(1) Production of Mixed Metal Oxide

Sodium carbonate (Na₂CO₃, produced by Wako Pure Chemical Industries,Ltd., purity: 99.8%), manganese (IV) oxide (MnO₂, produced by KojundoChemical Laboratory Co., Ltd., purity: 99.9%), iron(II, III) oxide(Fe₃O₄, produced by Kojundo Chemical Laboratory Co, Ltd., purity: 99%)and nickel(II) oxide (NiO, produced by Kojundo Chemical Laboratory Co.,Ltd., purity: 99%), as metal-containing compounds, were weighed toobtain an Na:Mn:Fe:Ni molar ratio of 0.7:0.42:0.42:0.16, and mixed in adry ball mill over 4 hours to obtain a mixture of the metal-containingompounds. The obtained mixture of the metal-containing compounds wasfilled in an alumina boat, heated in an air atmosphere by using anelectric furnace, and kept at 800° C. over 2 hours to obtain Mixed MetalOxide E10 of Example 10. FIG.9 shows the powder X-ray diffractionpatternof Mixed Metal Oxide E10 of Example 10.

(2) Evaluation of Charge/Discharge Performance as Positive ElectrodeActive Material of Sodium Secondary Battery

A battery was produced by using Mixed Metal Oxide E10 of Example 10 asthe positive electrode active material for sodium secondary batteries,and subjected to a constant current charge/discharge test under the sameconditions as in Comparative Example 1. Charge/discharge of this batterywas repeated 10 cycles, and the discharge capacity maintenance rate for10th cycle was 84% based on the discharge capacity for 1st cycle.

Example 11 (Na:Mn:Fe:Ni=0.7:0.46:0.46:0.08; in Formula (1), M¹ is Mn, Feand Ni, a is 0.7, and b is 0.46)

(1) Production of Mixed Metal Oxide

Sodium carbonate (Na₂CO₃, produced by Wako Pure Chemical Industries,Ltd., purity: 99.8%), manganese(IV) oxide (MnO₂, produced by KojundoChemical Laboratory Co., Ltd., purity: 99.9%), iron(II, III) oxide(Fe₃O₄, produced by Kojundo Chemical Laboratory Co., Ltd., purity: 99%)and nickel(II) oxide (NiO, produced by Kojundo Chemical Laboratory Co.,Ltd., purity: 99%), as metal-containing compounds, were weighed toobtain an Na:Mn:Fe:Ni molar ratio of 0.7:0.46:0.46:0.08, and mixed in adry ball mill over 4 hours to obtain a mixture of the metal-containingcompounds. The obtained mixture of the metal-containing compounds wasfilled in an alumina boat, heated in an air atmosphere by using anelectric furnace, and kept at 800° C. over 2 hours to obtain Mixed MetalOxide E11 of Example 11. FIG. 10 shows the powder X-ray diffractionpattern of Mixed Metal Oxide E11 of Example 11.

(2) Evaluation of Charge/Discharge Performance as Positive ElectrodeActive Material of Sodium Secondary Battery

A battery was produced by using Mixed Metal Oxide E11 of Example 11 asthe positive electrode active material for sodium secondary batteries,and subjected to a constant current charge/discharge test under the sameconditions as in Comparative Example 1. Charge/discharge of this batterywas repeated 10 cycles, and the discharge capacity maintenance rate for10th cycle was 90% based on the discharge capacity for 1st cycle.

Production Example 1 (Production of Porous Laminated Film)

(1) Production of Slurry of Applying Heat-Resistant Porous Layer

After dissolving 272.7 g of calcium chloride in 4,200 g of NMP, 132.9 gof para-phenylenediamine was added and completely dissolved therein. Tothe obtained solution, 243.3 g of terephthalic acid dichloride wasgradually added to effect the polymerization, and thereby obtain apara-aramide. The obtained solution was further diluted with NMP toobtain a para-aramide solution having a concentration of 2.0 wt %. To100 g of the obtained para-aramide solution, 2 g of a first aluminapowder (Alumina C, produced by Nippon Aerosil Co., Ltd., averageparticle diameter: 0.02 μm) and 2 g of a second alumina powder(Sumicorundum AA03, produced by Sumitomo Chemical Co., Ltd., averageparticle diameter: 0.3 μm), as a filler in total of 4 g, were added andmixed. The resulting mixture was subjected to a nanomizer three times,filtered with a 1,000-mesh metal screen, and defoamed under reducedpressure to produce a slurry of applying for heat-resistant porouslayer. The amount of the alumina powder (filler) was 67 wt %, based onthe total weight of the para-aramide and alumina powder.

(2) Production of Porous Laminated Film

As for a porous film, a polyethylene porous film (film thickness of 12μm, air permeability of 140 seconds/100 ml, average pore diameter of 0.1μm, void content of 50%) was used. The polyethylene porous film abovewas fixed on a 100 μm-thick PET film, and the slurry of applying forheat-resistant porous layer was applied on the porous film by a barcoater manufactured by Tester Sangyo Co., Ltd. The coated porous film onthe PET film was, while maintaining the integrity, dipped in water,which is a poor solvent, to precipitate a para-aramide porous film(heat-resistant porous layer). After that, the solvent was dried and thePET film was removed to yield a porous laminated film in which aheat-resistant porous layer and a porous film were stacked each other.The thickness of the porous laminate film was 16 μm, and the thicknessof the para-aramide porous layer (heat-resistant porous layer) was 4 μm.The air permeability of the porous laminate film was 180 seconds/100 ml,and the void content was 50%. The cross-section of the heat-resistantporous layer in the porous laminated film was observed by a scanningelectron microscope (SEM), as a result, the heat-resistant porous layerwas found to have relatively small pores of approximately 0.03 μm to0.06 μm and relatively large pores of approximately 0.1 μm to 1 μm.Evaluations of the porous laminated film were performed as in thefollowing (A) to (C).

(A) Thickness Measurement

The thicknesses of the porous laminated film and the porous film weremeasured in accordance with JIS standards (K7130-1992). The thickness ofthe heat-resistant porous layer was determined by subtracting thethickness of the porous film from the thickness of the porous laminatedfilm.

(B) Measurement of Air Permeability by Gurley Method

The air permeability of the porous laminated film was measured based onJIS P8117 by a digital-timer type Gurley densometer manufactured byYasuda Seiki Seisakusho, Ltd.

(C) Void Content

The obtained porous laminated film sample was cut into a square shapewhich is 10 cm on each side, and the weight W (g) and the thickness D(cm) were measured. The weight (Wi (g)) of each layer in the sample wasdetermined, the volume of each layer was determined from Wi and the truespecific gravity (true specific gravity i (g/cm³)) of each layer, andthe void content (vol %) was determined according to the followingformula:Void content (vol %)=100×{1−(W1/true specific gravity 1+W2/true specificgravity 2+ . . . +Wn/true specific gravity n)/(10×10×D)}

When the porous laminated film obtained by Production Example is used asa separator in the sodium secondary batteries of the above Examples, thesodium secondary batteries can more successfully prevent thermal filmrupture.

The invention claimed is:
 1. A mixed metal oxide represented by thefollowing formula (1):Na_(a)M¹O₂   (1) wherein M¹ represents Mn, Fe and Ni, and a is a valuefalling within the range of from 0.7 to 0.9, and wherein a M¹:Mn molarratio is 1:b, and b is a value not less than 0.333 and 0.46 or less,wherein a compositional ratio by mol of Mn:Fe:Ni is 1:x:y, wherein x is1 and y is 0.17 to
 1. 2. A positive electrode active material for sodiumsecondary batteries which comprises the mixed metal oxide according toclaim
 1. 3. A positive electrode for sodium secondary batteries whichcomprises the positive electrode active material according to claim 2.4. A sodium secondary battery having the positive electrode according toclaim
 3. 5. The sodium secondary battery according to claim 4 furtherhaving a separator.
 6. The sodium secondary battery according to claim5, wherein the separator is a separator having a porous laminated filmin which a heat-resistant porous layer comprising a heat-resistant resinand porous film comprising a thermoplastic resin are stacked to eachother.